With the advances of semiconductor and computer technology, computer systems are becoming faster and at the same time smaller in size. Desk-top and even lap-top computer systems now possess processing speeds of main-frame computers that used to fill up a small room. Even hand-held computer systems such as personal digital assistants (PDA), which are becoming more popular, are getting more powerful. As computer systems become more powerful and more miniaturized, power-conservation presents a difficult challenge to overcome. Because of their small size, hand-held computer systems are powered by batteries which have limited operating duration. Since more power is required for faster and more powerful processors, innovative solutions are often required to conserve power and thereby extend battery operating duration. To conserve power for hand-held computer systems, a common method is to introduce low-power operating modes for these systems.
Reference is now made to Prior Art FIG. 1 illustrating a block diagram of a typical hand-held computer system 10. Hand-held computer system 100 includes main controller 10, dynamic random access memory (DRAM) 100, liquid crystal display (LCD) 102, keyboard 103, and a communications device such as modem 104. Main controller 101 comprises central processing unit (CPU) 105, DRAM controller 106, LCD controller 107, and operating mode controller 108 which are linked together by internal bus 109. CPU 105 oversees the operations of DRAM controller 106, LCD controller 107, operating mode controller 108, as well as the peripheral devices namely keyboard 103 and modem 104. DRAM controller 106 controls data transfer between DRAM 101 and main controller 100. LCD controller 107 receives display data from DRAM controller 106 and in-turn provides the data to LCD 102 for display. Operating mode controller 108 performs power management by controlling the operating mode of main controller 100.
Hand-held computer systems may have three operating modes: NORMAL, IDLE, and STANDBY. In a NORMAL operating mode, a hand-held computer system executes a task and therefore requires all of its hardware elements namely the central processor (CPU), the liquid crystal display (LCD), the universal asynchronous receiver/transmitter (UART) unit, the oscillator etc. to be fully functioning. During the NORMAL operating mode, power is consumed is the full operating power.
An IDLE operating mode is a low-power mode. In an IDLE operating mode, the hand-held computer system functions but awaits for the next event (i.e., activity) such as a key press to generate an interrupt. Another event, for example, might be the receipt of data at the input port. In the IDLE operating state, all the hardware elements remain functioning, except for the CPU. During the IDLE operating mode, power consumed is about 25 percent of the NORMAL operating mode.
A STANDBY operating mode is the lowest power mode. In a STANDBY operating mode, the system has been inactive for a relatively long period of time and is therefore switched "off". In this mode, except for the wake-up circuitry, everything else including the LCD, the CPU, and the oscillator are turned off. During the STANDBY operating mode, power consumed is about 1 to 2 percent of the NORMAL operating mode.
During the STANDBY operating mode, LED display 103 is shutoff and DRAM controller 106 is put in a self-refresh mode to recharge the dynamic memory cells of DRAM 101 to retain the memory content. For some hand-held systems, it is desirable to have limited display at lower resolution during a new low-power operating mode. The limited display may be used, for example, to indicate the status information of the hand-held system. At the same time, it is desirable that the level of power consumption is minimized. This presents a challenge because under the Prior Art, providing display requires DRAM controller 106, LCD controller 107, LCD 102, and DRAM 101 to be turned on which results in excessive power consumption given the limited display capability desired for the STANDBY operating mode.
Thus, a need exists for an arrangement adapted to accommodate a low-power operating mode having reduced display capability for hand-held computer systems.